Reshaping of the cornea, for refractive vision correction, has been the object of various procedures, some of which have only recently been developed. In one well known procedure (radial keratotomy-RK), the cornea is incised with radial cuts to flatten the anterior surface shape of the cornea in order to correct for myopia. This procedure, is however a surgical one, requiring a high degree of skill and judgment for effective and safe implementation. Additionally, the myopia-corrective flattening is usually not stable, even when properly done, with gradual progression to hyperopia over time.
In other, more recently developed procedures, a pre-selected portion of the anterior surface of the cornea (i.e. corneal tissue) is removed to change the effective curvature of the cornea with respect to image focusing. The change in cornea curvature is selected to provide the requisite refractive vision correction.
A relatively recently developed excimer laser-based system operates using a photochemical ablation, rather than by cutting. The sequence of incident laser pulses gradually removes the corneal tissue in successive steps. This method known as photo-refractive keratectomy (PRK) is generally safe and effective. However, there are several drawbacks, in addition to the high cost of the equipment, inherent with the PRK procedure. Foremost of the drawbacks is the error factor, or lack of emmetropia, of more than .+-.0.5 diopters, as compared to the less than .+-.0.25 diopter error, typical with spectacles or contact lenses. In addition, use of the laser results in a rough corneal surface. In addition, there are long term effects relative to the physiology of the cornea and its interaction with the laser during ablation, which may result in the gradual reversal of the correction or which provide complications due to wound healing and concern about possible mutagenic effects.
The cornea comprises a thin protective epithelium layer on top of the Bowman's membrane or layer, which in turn covers the major corneal stroma. While the epithelium is regenerative, the Bowman's membrane is not. With ablative corneal tissue removal procedures such as PRK, the epithelium and Bowman's membrane are removed together with a portion of the stroma. Subsequently, the epithelium regenerates on the exposed outer surface of the cornea but directly on the stroma, since the Bowman's layer is not regenerated. Direct regrowth of the epithelium on the stroma can however cause an undesirable corneal haze which gradually dissipates over time. PRK has not yet been approved by the FDA for use in the US.
Both RK and PRK, because of inherent instabilities and error factors, are also usually not suitable for correction of myopia of more than -6 diopters and PRK is not currently suitable for corrections other than myopia. A surgical procedure known as Automated Lamellar Keratoplasty (ALK) preserves the Bowman membrane and has been used for corrections of up to -20 diopters. In such procedure there is an initial surgical removal, with a micro-keratome, of a uniform thickness button or lenticule of corneal tissue of a thickness containing the epithelium layer, Bowman's membrane (intact) and a portion of the stroma. The button or lenticule preferably remains hingedly attached at one point to the cornea. The lenticule is moved out of the way, the stroma bed is then surgically reshaped, as required, and the lenticule is replaced, with good adherence and healing of the stroma--stroma surfaces and with the Bowman membrane being preserved, leaving the cornea clear. It appears that the stroma--stroma healing of the ALK procedure reduces, if not eliminates, wound healing instabilities, making this procedure the most suitable for large refractive corrections.
However, despite the advantage of retention of vision clarity and healing stability, the procedure is not very favored since it is complex, requiring high surgical skill, is expensive, is usually inaccurate, with dependency on the surgeon's skill, and it can cause irregular astigmatism. These factors can be attributed to the viscous nature and relatively generally unsupported character of a cornea, in addition to reflexive eye movements, which makes use of a scalpel, or even a micro-keratome, difficult and highly subject to inaccuracies.
It is an object of the present invention to provide a method and device for the highly controlled cutting removal of corneal tissue for refractive correction.
It is a further object of the present invention to provide a method and device for refractive vision correction, which embodies the advantages of the ALK procedures but with enhanced accuracy and reduced complexity.
It is a further object of the present invention to provide such method and device with an accuracy at least comparable to that of spectacles or lenses and wherein the smoothness, polish and clarity of original corneal tissue is substantially retained.